Update customizing.txt

The origin statement "which could be ... or whatever" **misguides** many newbies like me.

In fact, the ``login`` function in ``contrib.auth`` stores ``user.pk`` in session, then ``get_user`` function in ``contrib.auth`` gets ``user.pk`` in session and then passes it to your custom ``get_user`` as ``user_id``.

Which means, ``user_id`` prarameter in your custom ``get_user`` has to be the primary key of ``User`` object, too.
Fixed a regression in the test runner loading of runtests.py.

Refs #17365, #17366, #18727.
Properly force bytes or str for bcrypt on Python3
Fixed a regression from e23a5f9a4730ddecb8f3950ee2936716f458c506.

Excluded postgis specific gis tests from other spatial databases.

Refs #17365, #17366, #18727.
Merge pull request #1052 from dstufft/bcrypt-python3

BCrypt on Python3
Don't hardcode primary keys in gis tests.
Fixed embarrassing typo.
Fixed #20249 - Removed a "feature" in the tutorial that doesn't actually work.

Thanks bmispelon for the report and draft patch.
Fixed #20136 - Fixed and expanded the docs for loaddata and model signals.

Thanks brandon@ and Anssi for the report.
Fixed #20397 - Cleaned up issue with quotation marks in documentation
Updated test failure example.
Merge pull request #1058 from svisser/ticket_20397

Fixed #20397 - Cleaned up issue with quotation marks in documentation
Fixed #20398 - Added language selection code to example in documentation

Thanks ggbaker for the suggestion and Simeon Visser for the patch.
Merge pull request #1053 from makto/patch-1

Fixed #20394 - Clarified argument of get_user in docs